1. Field of the Invention
This invention relates to non-destructive test apparatus, and in particular to a probe for evaluating the intrinsic electromagnetic properties of a dielectric/magnetic structure.
2. Description of Related Art
The interaction between an electromagnetic wave and a dielectric/magnetic structure can be analyzed by reflecting the wave off of the structure and analyzing the reflected wave. A dielectric probe is essentially a conduit for directing electromagnetic waves at the surface of the dielectric/magnetic structure and directing the reflected waves to an analyzer which analyzes changes in the waveform. The analysis generally involves measurement of both phase and magnitude of the reflected wave. Because the amount of energy reflected or absorbed depends on the intrinsic electromagnetic properties of the structure, the intrinsic properties can be deduced from the magnitude and phase of the reflected wave. The physical and mathematical principles involved are well-known to those with a working knowledge of basic electromagnetic theory, and in particular of antennas which operate according to the same fundamental principles.
Conventional dielectric probes use a variety of conduits for directing the wave against the material including open-ended waveguides, open resonators, interdigital dielectrometers, and coaxial cables, all of which are capable of carrying high frequency waves such as microwaves and/or electromagnetic waves in the VHF/UHF band. As is well-known, the electric field vectors E and the magnetic field vectors H of the wave are respectively affected by the permitivity .epsilon. of the material and the permeability .mu. of material, both of which encompass a polarization component (.epsilon.', .mu.') and a loss component (.epsilon.", .mu."). The respective vector couplings change the relative magnitude of the E and H vectors, thereby changing both the overall phase and the magnitude of the wave.
Although the theory of electromagnetic wave interaction with a dielectric and/or magnetic material is in general well-known, the specific contributions of .mu. and .epsilon. are indistinguishable in the reflected wave by conventional measurement techniques, and thus one of the intrinsic properties .epsilon. and .mu. must be known in order to deduce the other.
This is not a problem in the case of a non-magnetic material because the permeability .mu. can be assumed to equal the free-space permeability constant. However, where magnetic effects are significant, another method of measuring .mu. is required. Conventional dielectric probes are incapable of measuring the intrinsic permeability of a material apart from its dielectric properties.
Knowledge about the permeability can be critical in a variety of situations. For example, the relative effects of .mu. and .epsilon. are important in evaluating the structure of relatively thin coatings because the thickness of maximum absorption or penetration is different for a non-magnetic and magnetic materials, as will be appreciated by those familiar with antennas. A purely dielectric material absorbs the greatest amount of energy at 1/4 the wavelength of the incident radiation, while a magnetic material absorbs the greatest energy at 1/2 the incident wavelength. Therefore, a complete analysis of any coating structure requires knowledge of .mu. , at least qualitatively, whenever the coating has magnetic properties.
It is of course possible to use conventional magnetic, as opposed to dielectric, probes to determine the permeability of a material which is magnetic. This is generally accomplished by reading changes in magnetic flux applied to a material and directed through a magnetic core transducer where it can be read by a sense coil. However, the use of separate magnetic and dielectric probes has a number of limitations, including difficulties in implementation and especially the inability of this .mu. measurement technique to be used at high frequency.